The present invention relates generally to a programmable trigger coupling circuit, and more particularly to a trigger circuit for generating a trigger pulse in response to a selected signal component of an input trigger signal.
Trigger circuits have been widely used in various electronic test and measurement instruments such as, for example, oscilloscopes, frequency counters, and universal counters/timers in order to actuate a circuit or initiate some predetermined signal. One primary function includes selection of an input signal portion to be displayed or measured.
To gain a better understanding of the present invention, a conventional trigger coupling circuit used in an oscilloscope will be described with reference to FIG. 1. An input signal to be displayed is applied to input connector 10. Connected to input connector 10 is a vertical channel for conditioning the input signal before driving the vertical deflection plates of cathode ray tube (CRT) 18, and such vertical channel includes input circuit 12, which typically consists of a signal coupling circuit, a multi-step attenuator and an input amplifier, delay line circuit 14, and vertical output amplifier 16. A horizontal deflection system includes mode selection switch 22, coupling circuit 20, trigger generator 26, sweep generator 30 and horizontal output amplifier 32. Mode selection switch 22 selects a triggering signal from either internal (INT) or external (EXT) sources. An external triggering signal is applied to external input connector 24. Trigger generator 26 generates a trigger pulse by comparing the triggering signal from trigger coupling circuit 20 with a controllable trigger level from trigger level control potentiometer 28.
In operation, the vertical channel drives the vertical deflection plates of CRT 18 with a push-pull deflection signal voltage of certain amplitude determined by attenuating and then amplifying the input signal to suitable levels. The horizontal deflection system generates linear ramp (sawtooth) signals of different rates to produce a timebase. The trigger generator circuit is used to actuate the ramp signal in synchronism with the input signal to be displayed on the CRT screen as a stable waveform.
One objective of trigger coupling circuit 20 is to preclude the possibility of undesired signal components of the trigger signal reaching trigger generator 26. The most simple oscilloscopes have only AC and DC coupling modes, but more sophisticated laboratory instruments have additional coupling modes such as low frequency reject (LF REJ) and high frequency reject (HF REJ) modes, such as is shown in FIG. 1. When coupling selection switch 34 is in AC coupling position A, coupling capacitor 36 is connected in series with the signal path to block DC and very low frequency components. This mode is particularly useful when triggering on small AC signal components superimposed with a large DC signal. A DC coupling mode is selected at position D of coupling selection switch 34 and useful for triggering on very low frequency signal or on a predetermined DC level of the input signal, since the entire signal is passed to trigger generator 26 in this mode.
At position B of coupling selection switch 34, LF REJ mode is selected in which the trigger signal is coupled to trigger generator 26 through capacitor 38 of relatively low capacitance, thereby attenuating the input signal components substantially below a predetermined low frequency. This mode is particularly useful when low frequency noise such as power supply ripple is included in the signal. HF REJ mode is selected at the position C of coupling selection switch 34 in which the trigger signal is coupled through capacitor 36 to a resistor 40 and a capacitor to ground (not shown). This mode is essentially the same as the AC mode with the exception of attenuating high frequency components over some predetermined value. This mode will stably trigger input signals with large amounts of HF noise, and display modulated signal waveforms.
Drawbacks of prior art trigger coupling circuits are the use of bulky coupling capacitor 36 in series with the signal path and difficulty in selecting different coupling modes by electronic means. The use of such a capacitor is an obstacle to the implementation of all oscilloscope circuits in one or a few integrated circuits (LSIs). Additionally, loss of bandwidth due to stray capacitance is a problem.